pgsql: Work around cross-version-upgrade issues created by commit 9e38c

Work around cross-version-upgrade issues created by commit 9e38c2bb5.

Summarily changing the STYPE of regression-test aggregates that
depend on array_append or array_cat is an issue for the buildfarm's
cross-version-upgrade tests, because those aggregates (as defined
in the back branches) now won't load into HEAD.  Although this seems
like only a minimal risk for genuine user-defined aggregates, we
need to do something for the buildfarm.  Hence, adjust the aggregate
definitions, in both HEAD and the back branches.

Discussion: https://postgr.es/m/1401824.1604537031@sss.pgh.pa.us
Discussion: https://postgr.es/m/E1kaQ2c-0005lx-Eg@gemulon.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5ce51f8280d3e2a59dbcd93c9d8244e3bf96944d

Modified Files
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src/test/regress/expected/polymorphism.out | 24 +++++++++++++-----------
src/test/regress/sql/polymorphism.sql      | 13 ++++++++-----
2 files changed, 21 insertions(+), 16 deletions(-)